The long term objective of this research program is to gain an understanding of functional pathways in the spinal cord in cellular terms in order to identify basic mechanisms involved in segmental motor control as well as how the neuronal and synaptic components of these pathways can adjust after injury. These studies will be carried out at the level of synaptic connections between single identified classes of neurons, e.g. single spindle afferent fibers and alpha-motoneurons, using simultaneous intracellular impalement of both cells. The major aims are to analyze the changes in transmission that occur during and after physiologically realistic programs of high frequency stimulation. Study of the amplitude modulation of EPSPs in different postsynaptic targets of the same afferent fiber in response to a frequency moduelated train of impulses in that fiber will provide an indication of how neurally coded information is transmitted across synapses and the nature and diversity of the filtering process. What factors are responsible for differences in short term plasticity observed at different Ia/motoneuron connections? Are they presynaptic or postsynaptic? Do different afferent or motoneuron "types" have different susceptibilities with regard to transmission during and after high frequency stimulation? Do such phenomena take place at other identified connections in the spinal cord, e.g. spindle afferent projection to cells in Clarke's Column? Are these mechanisms of importance in determining such key functions as orderly recruitment. Can these synaptic properties be modified after peripheral nerve injury? By bridging cellular and functional neurobiology these findings ar expected to provide a more comprehensive understanding of synaptic transmission in the spinal cords with relevance to how these pathways participate in motor control. These studies will also permit evaluation of how such functions are affected and undergo compensatory changes after neural injury.